I'd always assumed it was Palmer (he's mechanically inclined and could know how to pick a lock).

I've also heard arguments that it was Windows. It goes like this: Windows is the one you see in silhouette about to be thing-ed by the dog. Later, you hear him drop something when he comes across Bennings being thing-ed, which are the keys he just used to break into the blood bank. (Even though Windows was a thing himself, he was still genuinely surprised that the burned remains were alive.) Palmer's attack didn't turn him into a thing, it just forced him out in the open. MacReady's test didn't work on Windows because he had thought of it long enough beforehand that he was able to prepare for it.